D.I.Y. Vacuum Tube Guitar Electronics

Contents

Overview

Guitar amps are wonderful things, but they’ve often got this little problem. They’re loud. I got into building DIY amps because of that. My Music Man HD130 was just ridiculously loud. I mean, we’re talking jet airplane kind of loud here. Guitarists are constantly struggling with how to get great tone at lower volume. Amptone is pretty much dedicated entirely to that and has a huge collection of information on the subject.

One method of lowering the volume of a cranked amplifier is to place a device inbetween the output of the amp and the speaker or speakers it’s driving. This device is called a power attenuator. Its purpose is to send a reduced signal to the speaker, which will result in a lower volume, while allowing the amplifier to work as normal.

Resistive

L-Pad

An L-Pad is a very simple, purely resistive, voltage divider attenuator. It consists of 2 resistors, one in series with the speaker, and one in parallel with it. The ratio between the 2 forms a voltage divider. There’s a useful L-Pad calculator which you can plug an impedance and desired dB attenuation and it will spit out values for R1 and R2. Or, I’ve included a table below with the values necessary for -3 through -15 dB in 3 dB increments at 4 and 8 ohms.

I’ve heard it mentioned that purely resistive attenuation seems to sound fine for low power amps, but higher powered amps need a reactive load to sound more natural under heavy attenuation.
I built an L-Pad into the 8” speaker cabinet I built for my Moonlight. It has a single 8”, 4 ohm Jensen reissue speaker and a rotary switch allowing for the selection of no attenuation, -6dB, -9dB, -12dB, and -15dB. It’s quite effective. The higher attenuation settings do sound fairly weak to the ear, but that’s due in part to the fact that, when driven by a 1W amp, the SPL at -15dB is extremely low. I’ve excellent results close-miking the speaker for recording at -9dB and -12dB.

dB

Z

R1 (ohms)

R2 (ohms)

-3

4

1.17

9.70

-6

4

2.00

4.02

-9

4

2.58

2.20

-12

4

3.00

1.34

-15

4

3.29

0.87

-3

8

2.34

19.39

-6

8

3.99

8.04

-9

8

5.16

4.40

-12

8

5.99

2.68

-15

8

6.58

1.73

Bridged-T

A bridged-T attenuator is another purely resistive design, but one which reportedly has superior sonic characteristics to the L-Pad.

FM Systems, Inc. has a page about the bridged-T. I found the link on the Ampage BBS. In this circuit, the values of R3 and R4 are always equal to the characteristic impedance of the whole thing. I.e. if you’re building one for an 8 ohm load, R3 and R4 are both 8 ohms.

FM Systems’ page describes the math for figuring R1 and R2. I’ve included the same table below for the Bridged-T as above for the L-Pad.

dB

Z

R1 (ohms)

R2 (ohms)

-3

4

9.70

1.65

-6

4

4.02

3.98

-9

4

2.20

7.27

-12

4

1.34

11.92

-15

4

0.87

18.49

-3

8

19.39

3.30

-6

8

8.04

7.96

-9

8

4.40

14.55

-12

8

2.68

23.85

-15

8

1.73

36.99

Reactive

Weber M.A.S.S.

The Weber MASS attenuator is extremely clever, and by most reports very effective at attenuating an amp while retaining most of its character and responsiveness. It uses an actual speaker voice coil, without the cone, as a load for the amp. The amp reacts just like it would driving a speaker because it is driving a speaker, just one which produces no sound!

Weber sells complete attenuators and also just the “motor” as they call it, for DIY’ers who want to build their own. I’m considering building one into my Transatlantic build.

The MASS comes in 25, 50, and 100 watt versions, and also a 10 watt “Mini-MASS”. Weber’s online store also has pictures of the bare motor: 25, 50, 100.